Exploration of the earth to locate oil, minerals or other valuable deposits, uses seismic techniques in order to provide information about the subsurface structure of the earth so that an assessment can be made as to whether a particular deposit may or may not be present. In particular, seismic techniques provide an indication of various subsurface structures including porous sandstone or fractured carbonates which may contain oil or other valuable deposits.
Whilst this technique does provide an indication of subsurface structures, it is possible a particular structure which may appear to be of interest, in fact represents a significant hazard to drilling. For example, it is possible that subsurface volcanos exhibit very similar characteristics to anticlines which may contain oil. If a drilling operation is initiated and it is found that the anticline is in fact a volcano, then a considerable financial loss results because of the cost of establishing the drilling platform. This is particularly the case in relation to marine exploration because of the significantly higher drilling cost.
In order to provide more information concerning the nature of subsurface structures, magnetic data of the exploration area is obtained.
A magnetic gradient survey enables magnetic data relating to the survey region to be obtained and this magnetic data can be used to provide information relating to the nature of subsurface structures. If the magnetic survey is overlayed with the seismic survey, then structures which appear to be of interest from the point of view of the seismic survey, can be further considered in the light of the magnetic data so that a clearer indication can be formed as to whether the structure is a structure which may contain a deposit of interest such as an oil deposit, or whether the structure exhibits magnetic phenomena, such would be the case with a volcano. Thus, the location of drilling platforms can be decided with more precision to avoid subsurface structures which could be hazardous from the point of view of a drilling operation. However, conventional techniques for processing magnetic gradient data contain considerable distortion and anomalies due to unwanted magnetic effects including the ship bias referred to above.
The conventional method of obtaining that data in relation to onshore exploration is to tow magnetometers behind an aircraft to obtain magnetic data in relation to the survey area. In the case of marine exploration, the magnetometers are towed behind a ship.
The usefulness of marine magnetic data which is obtained in this manner has been limited by the quality of the data which is obtained. One of the major problems with marine magnetic data acquisition is interference or so-called ship bias which is created by the magnetic field induced by the ship which tows the magnetometers.
In conventional marine magnetic data acquisition, two magnetic field sensors, which are generally referred to as fish, are towed behind a ship. The magnetometers are connected to a tow line and the magnetometer which is closest to the ship is towed at a distance of some 300-600 metres behind the ship to avoid the effect of ship induced magnetic field. Furthermore, the magnetometers are separated by a distance of in excess of 100 metres. The reason for the length of the tow line and the separation of the magnetometers is to reduce ship bias and therefore provide data which is relatively free of that bias.
However, because the towing line is so long and the distance of separation so great, the degree of drift of the magnetometers in the ocean as the magnetometers are towed behind the ship is considerable. Furthermore, the processing techniques used to acquire magnetic gradient data assume that after a given time period, the trailing magnetometer will be towed to a position which coincides with a previous position of the magnetometer which is closest to the ship. This assumption is made during the processing of the magnetic data. However, because the tow line is extremely long and the distance of separation between the magnetometers is considerable, drift of the magnetometers due to sea currents and the like means it is unlikely that the trailing magnetometer will actually occupy the same position as the first magnetometer after that given time period.
Furthermore, in the acquisition of the magnetic data, the ship is required to travel along predetermined survey lines and because of the drifting of the sensors, the sensors do not actually travel along the survey line. In the southern hemisphere, if the magnetometers drift to the north of the survey line, the recorded gradient data between the two magnetometers will have larger distortion than if drifting towards the south of the survey line.